LEDs are used for a variety of lighting applications (e.g., full-color displays, lamps, traffic lights, etc.), and are increasingly finding additional applications as LED technology improves and the cost of LEDs decreases.
LEDs are fabricated using lithographic techniques, which include using alignment methods to aligning adjacent layers of the LED structure. Many LEDs include a rough upper surface with a surface roughness comparable to the wavelength of light generated by the LED. The rough surface allows light that is otherwise trapped by total internal reflection to escape the LED structure, thereby increasing the LED light output.
While the rough surface improves LED light output, it also interferes with the alignment imaging. Lithography requires precise alignment between an existing layer and a subsequent layer. Alignment is typically accomplished using pattern recognition techniques based on an alignment structures or “alignment marks.” In a preferred case, an image of wafer and reticle alignment marks respectively associated with the wafer and the reticle (mask) is captured by a Machine Vision System (MVS), such as described in U.S. Pat. No. 5,621,813. Typically, visible-wavelength light is used for alignment imaging. The alignment mark images are displayed so that an operator can check the relative alignment. The relative position of the alignment marks is used to adjust the relative position of the wafer and reticle of the lithography system until their alignment is established.
Unfortunately, the rough surface of the LED scatters the imaging light and degrades the quality of the MVS image (or the diffraction signal) used to carry out alignment. Accordingly, improved methods are needed to perform alignment when fabricating LEDs using lithographic techniques when the LEDs have a roughened surface that interferes with alignment mark imaging.